/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
         ListNode* fast = pHead;
        ListNode* slow = pHead;
        while(fast&&fast->next) {
            fast=fast->next->next;
            slow=slow->next;
            if(fast==slow) break;
        }
        if(!fast||!fast->next) return nullptr;
        fast=pHead;
        while(fast!=slow) {
            fast=fast->next;
            slow=slow->next;
        }
        return slow;
    }
};